Document how to call an uncurried function with a single `unit` argument

[ryyppy]

This is a very common issue for learners (see this SO question).

We should give some pointers in the uncurried function section on how to call an uncurried function with a single unit argument:

E.g. this is a way to do it:

let echo = (. a) => a
let _ = echo(. ignore())

[ryyppy]

Fixed in #166